1. Field of the Invention
This invention relates to a resin composition having improved dynamic properties. More specifically, it relates to a resin composition of improved dynamic properties comprising an aromatic copolyester, a polyamide and an ionomer.
2. Description of the Prior Art
Aromatic copolyesters prepared from terephthalic acid and isophthalic acid or its functional derivative and a bisphenol and its functional derivative are thermoplastic resins having thermal stability with a high heat distortion temperature and a high heat decomposition temperature which also have superior mechanical and electrical properties. They find wide applications in fields which require high performance as engineering plastics. They, however, have the defect that their moldability is inferior to general thermoplastic resins because of their high heat distortion temperature. For example, they require high molding temperatures and high injection pressures in injection molding, and have the defect that molded articles prepared therefrom tend to have sinks, flow marks, large internal stresses, etc.
Generally, sinks, flow marks, large internal stresses, etc. greatly influence not only the appearance of the molded articles but also their mechanical properties, and frequently, the superior properties inherent in the resin are not exhibited fully. Furthermore, because depending upon the type of the molded articles, a sufficient flow length in the molding cavity cannot be obtained, the molded article will sometimes not be perfect. In this sense, superior moldability as well as various superior performances have previously been considered as important factors in evaluating a certain resin as engineering plastics. It is known, on the other hand, that when it is desired to improve the moldability of thermally stable engineering plastics by blending other polymers, the heat distortion temperature tends to be lowered.
Aromatic copolyesters have the many advantages described above, but because they are not resistant to organic solvents, their uses are limited.
On the other hand, polyamides have very good organic solvent resistance, abrasion resistance and moldability, but insufficient thermal stability. For example, polyhexamethylene adipamide has a heat distortion temperature of 59.degree. C. and polycaprolactam has a heat distortion temperature of 56.degree. C. Another serious defect of the polyamides is that they have high moisture absorption and as a result their dimensions and dynamic properties change, and their electrical properties deteriorate, upon the absorption of moisture.
A resin composition composed of an aromatic copolyester and a polyamide has a markedly increased heat distortion temperature over the polyamide alone, and improved moldability and solvent resistance over the aromatic copolyester. The degree of improvement of these properties are far higher than those which would be expected from the mere addition of the two ingredients. That is the improved properties are unexpected from the properties of the individual ingredient of the composition. Such a resin composition is very useful because it retains the superior tensile strength, flexure recovery characteristics and dimensional stability possessed by the aromatic copolyester and the superior crack resistance and abrasion resistance possessed by the polyamide. In this regard, see Japanese patent application (OPI) Nos. 4146/75 and 98765/77 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".).
When a resin composition comprising an aromatic copolyester and a polyamide is molded into various molded articles by injection molding and their properties are examined in detail, the following defects are found. Specifically, the thin parts, such as in the gate portion (See 2 in FIG. 1), and protruding slender rod-like portions of the molded articles are susceptible to breakage especially in an absolutely dried condition, and release of the molded articles from the mold is hampered, and the design of the product is restricted. Furthermore, it has been ascertained that even those molded articles which have a relatively large thickness are susceptible to cracking upon the application of an impact force such as falling impact or falling ball impact. These phenomena drastically restrict the utility of these resin compositions.
Furthermore, while the two-component resin composition has reduced moisture related changes in dimension or electrical properties as compared with the polyamide, it is frequently unsatisfactory for use as various machine parts or electric component parts which require a high dimensional accuracy. Hence, it is desired to improve the resin composition further.
In such applications as electric wires and cables or electrically insulating tubes, a composition composed of an aromatic copolyester and a polyamide is rated high because it has improved electrical properties based on a decrease in moisture absorption, increased thermal stability and superior cut-through property (i.e., it is difficult to cut through the tube when an acute-angled body is urged against it). On the other hand, it has been frequently desired to improve the flexibility, dimensional stability to water, electrical properties, elongation and impact properties of the resin composition. Furthermore, in the case of extruded pipes, tubes, monofilaments, sheets, and films, products having high levels of dynamic properties, flexibility, thermal stability and solvent resistance have been desired.
The ionomer is known (Japanese Pat. No. 6810/64). A composition composed of a polyamide and the ionomer is also known. This composition has superior dynamic properties to the polyamide alone, and has improved extrusion moldability and blow moldability, and improved resistance to water or organic solvents over that of the polyamide, but it has a lower heat distortion temperature in comparison to the polyamide alone (Japanese Pat. No. 12546/67).
A composition comprising an aromatic copolyester and an ionomer is also known (Japanese patent application (OPI) No. 51244/78). This resin composition has a longer flow length in the molding cavity during injection molding than the aromatic copolyester alone, but when caused to reside within a molding machine, tends to decompose. Thus, the properties of the resin composition are not fully exhibited in the resulting molded articles.